The development of optical coherence tomography (OCT) in recent years has concentrated on Fourier domain (FD) techniques for high speed cross-sectional imaging of biological tissue. Namely, FD techniques provide increased signal-to-noise ratio (SNR) and increased robustness over traditional OCT techniques. The SNR advantage of FDOCT techniques may be employed for faster image acquisition, thereby enabling practical three-dimensional OCT imaging in living subjects. In FDOCT, the locations of scatterers within a sample are obtained by Fourier transformation of real-valued spectral interferograms, which are generated by mixing light backscattered from the sample with reference light. The Fourier transform of the interferogram is Hermititan symmetric, thereby introducing a complex conjugate artifact in which positive and negative distances are not resolved. In practice, this symmetric artifact may be avoided by locating the sample entirely within the positive or negative displacement range, thus utilizing only one half of the potential total imaging depth. Such one sided imaging is suitable for thin objects, but imaging of extended objects is limited by the characteristic roll-off in sensitivity that is typically associated with the finite spectral resolution of FDOCT systems.
Full-range imaging, in which positive and negative distances are resolved, can be achieved by indirectly measuring the complex component of the interferometric OCT signal using techniques borrowed from phase shift interferometry. The imaginary component of the interferogram is obtained by shifting the phase of the reference reflection in increments of 90 degrees. Phase shifting has been demonstrated in spectrometer-based FDOCT systems using a discretely stepped piezoelectric transducer (PZT) mounted reflector or an electro-optic modulator in the reference arm. One drawback of sequentially shifting the interferogram is that significant image corruption results from small deviations (e.g., chromatic deviations) in the actual phase shift obtained or from small (i.e., sub-wavelength) sample motion between the phase shifted acquisitions.
Recently, the instantaneous acquisition of two phase shifted signals was demonstrated using linearly polarized light. This technique was limited by having only two phase shifted signals (which limits complex signal reconstruction) as well as by potential image corruption present in birefringent samples. Methods to instantaneously acquire three phase-shifted interferograms using 3×3 fused fiber couplers for both spectrometer-based and swept source (SS) FDOCT systems have also been employed. However, the performance of these types of systems for complex conjugate image reconstruction may be limited by the wavelength dependence of the splitting ratios associated with the fiber couplers. In addition, an approach to full range imaging based on frequency shifting may be considered, but this method would not compatible with spectrometer based systems.
Notably, all of the aforementioned phase shifting FDOCT techniques suffer image corruption arising from the mis-calibration of the phase shifts as well as from the wavelength dependence of the phase shifter. Numerical techniques to improve the suppression of symmetric artifact by compensating for the phase shift irregularities and for by accounting for axial sample motion in between phase shifted acquisitions have been previously been presented. However, there have not been any methods that address the removal of the complex conjugate artifact present in FDOCT images.
Accordingly, there exists a need for an effective method for eliminating the complex conjugate artifact in FDOCT images.